1. Field of Invention
The present invention relates to a power switch series connection technique. More particularly, the present invention relates to a design of a power switch series circuit and a control method thereof.
2. Description of Related Art
Modern high voltage and high power converters require a power device having a high withstand voltage value. However, a single power device cannot meet this requirement. In one solution of the prior art, a plurality of power switch devices are directly connected in series. For example, the power switch device may be a thyristor, a GTO (Gate Turn-Off Thyristor) or an IGBT (Insulated Gate Bipolar Transistor), which may be applied in electronic equipment such as a high-voltage DC circuit, a SVC (Static Var Compensator) or a high-voltage inverter.
However, when a conventional device such as the thyristor or the GTO is employed for series connection, the power consumption is large and the switching speed is slow, and as a result, such a conventional device cannot be widely applied in a PWM current transformation apparatus. In comparison, the IGBT, which is a semiconductor power device introduced in the 1980's, has a voltage control input characteristic and a low-impedance on-state output characteristic, and the switching speed of the IGBT is fast. Consequently, the IGBT can fully satisfy the requirement of the PWM current transformation technique. On the other hand, although the IGBT index and the gate control signal circuit in the series circuit are totally the same, due to the fact that each IGBT has a different performance and switching speed, the parameters of the elements of the gate control signal circuit are not consistent and the circuit has distributed inductance and distributed capacitance, the switching actions of the IGBTs are not consistent. A negative consequence of this is that a voltage between a collector and an emitter exceeds a rated withstand voltage value when an individual IGBT device is switched, thereby damaging the power device.
In view of the above, many in the industry are endeavoring to find ways in which to improve the power switch series circuit of the prior art, so as to quickly protect the power switch when an abnormal situation is encountered with the circuit, thereby improving the operation reliability of the series circuit.